The whole modern copyright issue is interesting. I never heard a word about it when I was trading cassette copies of albums, or making mixed tapes with my friends in the 80's and 90's. They just quietly introduced a tax on cassette tapes to cover the cost of piracy. Now peer to peer and the internet have introduced a much more efficient way to do the same thing. Only difference being that you don't actually have to go over to your friends house and tape his album. In fact you don't even have to know the guy. So I can see how it would impact sales. Fewer original purchases are required to distribute to a much broader audience. It used to be that you bought the albums the you liked most, while your buddies did the same and you broadened your taste by taping your second choices off of your friends. In fact, often they'd move up onto your first choice list and you'd end up buying their next album.
Anyhow...
The problems I see with someone selling roms are:
He is probably not assicated with the work to have the rom emulated. It's like selling something he didn't make or own.
Someone selling roms for profit is likely to attract attention and cause a legal crackdown. Game companies are getting to be bigger and bigger business and retro gaming is getting popular. Once the game giants realize that they can just repackage old games and make a million with no overhead, they'll want to shut down their competition. But I think it will take an even bigger spike in interest in retro gaming than we've seen so far, and it will take a few major sellers doing a big business to attract attention.
Basically we should be mad at someone selling for profit because it's against "honour among theives". Someone else did a lot of work to make the game available illegally for free and he's profiting off of someone elses work and good nature, which is scummy (scummier).
It also jeoprodizes the "preservationist" excuse we all use to justify posessing illegal roms.
I'm not sure how the arcade industry worked in it's infancy, but if the license for the title was anything like modern day EULA's (which I suspect it wasn't). You probably don't have any legal right to the licence if you bought the PCB second hand. So even the guys who think they're doing things legally, possibly aren't. (I'd be glad to be corrected on this one. I'd like to find out more how licensing worked in the arcade industry). I know that with often pirated software, today, like 3dsmax, you could buy a retail copy in box with all the manuals etc. second hand and you have no legal right to the license, you'll get no support from Autodesk, and no upgrade rights since you don't acutally puchase the software, you just pay for the right to use it and the EULA says it's non transferable.